Passive intermodulation (PIM) is a form of nonlinear distortion that is encountered in a growing number of communications and sensing systems. This distortion is created in passive RF system components such as coaxial connectors, antennas and filters as a result of small nonlinear characteristics of such passive components, and frequently transfers energy from high-power transmit signals to frequencies within the system's receive band, masking low-level receive signals or even saturating sensitive receive circuitry. See, for example, P. L. Lui, “Passive Intermodulation Interference in Communication Systems,” Electronics & Communication Engineering Journal, Vol. 2, June 1990, pp. 109-118. This reference is incorporated herein by reference along with all other references cited herein. Because the distortion products are generated after low-level receive signals are already present in the network, the distortion power cannot be removed by filters and arrives at the receiver along with the desired receive signals.
As a result of the great difference in power between transmitted and receive signals in a communication system, passive intermodulation distortion levels as low as −150 dBc are potentially problematic sources of interference in many systems as the nonlinearity of passive components causes power at transmit frequencies to mix into the system's receive band. Passive intermodulation is most problematic in transmit/receive systems where transmit and receive bands are closely spaced. Communication frequency bands are becoming more densely populated, making passive intermodulation a growing concern in the wireless community, cell phone applications representing one example.
Because PIM distortion cannot usually be mitigated by conventional means such as frequency filtering, many studies have been undertaken to identify the causes of PIM. See, for example, the above-referenced paper by Lui as well as the following papers: M. T. Abuelma'atti, “Prediction of Passive Intermodulation Arising From Corrosion,” IEE Proceedings, Science, Measurement and Technology, Vol. 150, No. 1, 2003, pp. 30-34; F. Arazm, “Nonlinearities in Metal Contacts at Microwave Frequencies,” IEEE Transactions on Electromagnetic Compatibility, Vol. EMC-22, August 1980, pp. 142-149; and J. Wilcox and P. Molmud, “Thermal Heating Contribution to Intermodulation Fields in Coaxial Waveguides,” IEEE Transactions on Communications, Vol. 24, No. 2, February 1976, pp. 238-243. PIM is known to occur at junctions of dissimilar metals and at junctions of metals and oxides. The unsoldered metal-metal junction in a coaxial connector is a major contributor to PIM in many microwave networks. Ferromagnetic conduction metals such as iron or nickel are also well known causes of PIM distortion, especially in coaxial connectors. See, for example, the above-referenced paper by Lui as well as the following references: J. Henrie, A. Christianson, and W. J. Chappell, “Prediction of Passive Intermodulation From Coaxial Connectors in Microwave Networks,” IEEE Transactions on Microwave Theory and Techniques, Vol. 56, No. 1, January 2008, pp. 209-216; and J. C. Pedro and N. B. Carvalho, Intermodulation Distortion In Microwave And Wireless Circuits, Artech House, Boston, Mass., 2003. As a result, efforts have been made to solve the problem through the choice of contact materials, including combinations of base material and plating. For example, low-PIM connectors are available in which the contacts are silver-plated or gold-plated with no nickel undercoat. However, there are tradeoffs with such connectors, most notably a higher cost of materials and manufacturing, and a shorter lifetime in some cases, in exchange for the lower PIM. Low PIM connectors are relatively expensive, bulky, low-bandwidth, and more susceptible to some environmental factors because of their composition. In addition, a need exists for even lower PIM in coaxial connectors and other passive RF components.